1. Field of the Invention
The present invention relates, in general, to an insulating material for a printed circuit board (PCB). More particularly, the present invention relates to an insulating material for a PCB, which includes a liquid crystal polyester resin and ceramic powder to exhibit superior dielectric properties and a small change in dielectric constant depending on changes in temperature so as to realize excellent reliability when applied to high-frequency circuits.
2. Description of the Related Art
Generally, in proportion to an increase in the speed and capacity of an electronic apparatus, the frequency of a signal, which is transmitted along the line of a PCB, is increased. To transmit the high-frequency signal without loss, such as signal reflection, impedance matching of an input terminal and an output terminal is regarded as very important.
As operating frequencies are increased to high frequencies, in particular, to the GHz (Gigahertz) level, almost all energy components of the signal between a signal line and a GND (ground) are transmitted in the form of an alternating current field. Accordingly, when the height or dielectric constant of an insulating material (the dielectric of a substrate) between the signal line and the GND is exactly uniform, signal loss, such as signal reflection, attributable to the difference in impedance, may be minimized, and furthermore, signal integrity may be assured.
In this regard, an epoxy resin, for example, FR-4, which is mainly used as the insulating material of a PCB, is greatly changed in dielectric constant depending on the change in temperature, as illustrated in FIG. 1, and therefore has −91897 ppm at −55° C. and 81712 ppm at 125° C., relative to the dielectric constant at 25° C. From this, a TCC (Temperature Coefficient of Capacitance) is determined to be 960 or more ppm/° C. The great change in dielectric constant of the epoxy resin depending on the temperature is considered to be a limitation in the ability to design a circuit of a PCB for transmitting the high-frequency signal in consideration of impedance matching. The dielectric constant of an electronic apparatus changes depending on the surrounding environment, in particular, temperature, and thereby the impedance of the matched signal line varies, undesirably causing signal loss, such as signal reflection.
In order to solve problems due to the impedance mismatch of the signal line upon the transmission of the high-frequency signal, the development of an insulating material for a PCB having a stable dielectric constant, that is, a low TCC (temperature coefficient of capacitance), depending on the change in temperature, is being extensively conducted these days in universities and research institutes, and by some advanced substrate manufacturers.
To date, an insulating material for high-frequency signal transmission has been actively developed according to the trend toward a low dielectric constant for increasing a transmission speed and a low dissipation factor for minimizing signal loss. The development of the insulating material for high-frequency signal transmission has progressed to the preparation of a resin having a dielectric constant and a dissipation factor lower than conventional epoxy resins and to the improvement of dielectric properties through the dispersion/addition of a high-frequency ceramic filler.
In the former case, research into the addition of conventional epoxy with PPE (PolyPhenylene Ether) or the formation of an epoxy structure into an interpenetrating network has been conducted. However, in the case where epoxy is added with PPE, it is difficult to realize a prepreg because the PPE is a thermoplastic resin, and an appropriate solvent is difficult to select. Further, methods for the formation of the interpenetrating network structure and the addition of a high-frequency ceramic filler suffer because the TCC cannot be sufficiently decreased.
To overcome the problems, attempts have been made to apply a functional polymer, including LCP (Liquid Crystal Polymer) or BCB (divinyl-siloxane-bis-benzocyclobutene), having high electrical properties, in particular, high dielectric properties, to PCBs.
BCB has a low dissipation factor and stable TCC properties, as illustrated in FIG. 2, but is disadvantageous because it is expensive and easily cracks in a curing reaction. Further, this polymer is not easily formed into a film, and is undesirably difficult to commercialize. Although the LCP has superior dielectric properties, including dielectric constant and dissipation factor, it is problematic in use in high-frequency circuits because the TCC thereof is as high as about 900 ppm/° C. at high temperatures (125° C.), as illustrated in FIG. 3.